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OF THE ACTION OF THE TEETH, ETC. 107
length. Old people, who have lost their Teeth, have not this
ridge. When in a sound state, the Gums are not easily
irritated by being wounded, and therefore are not so liable to
inflammation as other parts, and soon heal.
The Teeth being united to the Jaw by the Periosteum and
Gum, have some degree of an yielding motion in the living-
body. This circumstance renders them more secure ; it breaks
the jar of bony contact, and prevents fractures both of the
sockets and of the Teeth themselves.
OF THE ACTION OF THE TEETH ARISING FROM THE
MOTION OF THE LOWER-JAW.
The Lower-Jaw may be said to be the only one that has
any motion in mastication ; for the Upper-Jaw can only move
with the other parts of the head. That the Upper-Jaw and
head should be raised in the common act of opening the mouth,
or chewing, would seem, at first sight, improbable ; and from
an attentive view of the mechanism of the joints and muscles
of those parts, from experiment and observation, we find that
they do not sensibly move. We shall only mention one experi-
ment in proof of this, which seems conclusive : let a man place
himself near some fixed point, and look over it, to another
distant and immoveable object, when he is eating. If his head
should rise in the least degree, he would see more of the distant,
object over the nearest fixed point, which in fact he does not.
The nearer the fixed point is, and the more distant the object,
the experiment will be more accurate and convincing. The
result of the experiment will be the same, if the nearest point
has the same motion with the head ; as, when he looks from
imder the edge of a hat, or any thing else put upon his head, at
some distant fixed object. We may conclude then that the
motion is entirely in the Lower-Jaw : and, as we have already
described both the articulation and the motion of the bone, we
shall now explain the action of mastication, and, at the same
tune, consider the use of each class of Teeth.
With regard to the action of the Teeth of both Jaws, in
